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Photovoltaics Researchers Developing Self-Assembling Solar Cells

Date:

September 6, 2001

Source:

University Of Arizona

Summary:

Photovoltaics - the high-tech approach to converting sunlight directly into electricity - could be low cost and widely practical if based on organic "self-assembling" thin film technologies, say scientists at the University of Arizona.

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TUCSON, Ariz. -- Photovoltaics - the high-tech approach to converting sunlight directly into electricity - could be low cost and widely practical if based on organic "self-assembling" thin film technologies, say scientists at the University of Arizona.

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UA chemists and optical scientists have been funded by two separate new grants totaling more than $1 million to develop organic molecules that "self assemble," or self-organize, from liquid into efficient solar cell coatings.

They are now applying their discoveries and new materials to unconventional photovoltaics (PV) -- organic solar cell thin films. They are designing, synthesizing and characterizing molecules that will self-organize from solution into coatings about 100 nanometers thick, or about one-thousandth the thickness of a human hair. Molecules in the layer must be very highly ordered to efficiently transport electrical charge.

"What you'd really like is a solar panel array that would come on a flexible plastic substrate which would be extremely inexpensive and which you could roll out on your roof like wall paper," said chemistry Professor Neal R. Armstrong. "It would be efficient enough at energy conversion to economically generate power."

Armstrong is principal investigator on a 3-year, $490,000 grant from the Department of Energy's National Energy Research Laboratory (NREL). The grant is aimed at developing new organic "liquid crystal" PV materials that could be inexpensively wet-processed into large area panels.

Ninety-nine percent of the photovoltaic market today is based on single crystal silicon, an efficient and reliable but expensive material for solar cells. Silicon PV powers satellites and space missions, but cost, mainly, limits how much it does in meeting the world's energy demands.

According to Department of Energy statistics, Americans currently pay 6-to-7 cents per kilowatt-hour for conventionally generated electricity and 20-to-30 cents per kilowatt-hour for solar-generated electricity.

"That sounds discouraging, but the cost of solar-generated power was about 90 cents per kilowatt-hour 10 to 15 years ago," and can be expected to drop even more, Armstrong noted.

Efficient new inorganic thin films such as cadmium telluride and cadmium-indium-galleium-selenide are entering the PV market, he said, and their efficiencies and potentially lower cost are impressive. But the heavy metals, tellurium and selenium, in these PVs raise environmental concerns, both during their manufacture and ultimate disposal.

New organic solar cells would potentially be a less-toxic, Earth-friendly way to tap energy from the sun, he emphasized, because most of the target organic materials are environmentally benign when processed and when discarded in devices.

Armstrong was among hundreds of scientists world-wide interested in organic PV thin films when he joined the UA in the late 1970s. But until recently, these films were only one to one-and-a-half percent efficient at converting solar power to electrical power. There also were serious concerns about the long term stability of such thin organic films. Not surprisingly, funding agencies from the late 1980s to the mid-1990s weren't interested in organic solar cell research.

"The discouraging thing was that these materials could be tailored to absorb most of the solar spectrum, they were very cheap, and they were easy to spread as a thin film on a transparent electrode. But their electrical properties were very bad. Even if you could generate a lot of electrical charge inside a thin film you couldn't transport it, and you couldn't harvest it efficiently," Armstrong said.

"But the last decade has seen a big change in how people feel about organic technologies in general, and, specifically, about how you make them better electrical materials," he added.

Optical sciences Associate Professor Bernard Kippelen agreed. "Ten years ago, if you went to the Department of Energy and said you wanted to make an organic solar cell, they would have been very skeptical. But a lot of research has been done in the emerging field of organic electronics in the past 10 years, and it has completely changed the way people think."

Kippelen is principal investigator on a 3-year, $580,000 grant from the Office of Naval Research that involves Armstrong, Marder and Bredas . Their goal for Kippelen's grant is to develop self-assembling polymer (plastic) liquid crystals for lightweight, flexible and shock-resistant light-emitting diodes and devices needed by the military, as well as for PV use.

"Even a few years ago, people thought that any organic material for optical or optoelectronic application was unstable, that it could have only a very limited lifetime," Kippelen said. "With all the successful research that has been done now, people know that if you synthesize the materials correctly, if you purify them and keep them from water and oxygen, even organic materials can have very long lifetimes."

Industry now confidently markets "Organic EL" displays for car stereos and for cell phones, for example. These devices emit light using 100 nanometer-thick organic films that carry high current densities, have great stabilities, and are bright and pleasing to look at.

The challenge for PV is to achieve higher electrical "mobility," or create films that rapidly carry charge, Kippelen said.

"We don't want to make predictions that sound overly optimistic, but theoretically there is no reason that we cannot make solar cells with 20 percent efficiency," Kippelen said.

Researchers from the University of Cambridge and the Max Planck Institute reported in Science earlier this month that they have developed a potentially efficient self-assembled organic thin film PV, proving the concept works. Their work developed from an earlier collaboration with Bredas which demonstrated how the right kind of self-assembly would increase PV efficiency.

Initially, the UA scientists have focused on self-assembling liquid crystals that Armstrong and chemistry Professor David O'Brien developed from a common deep blue-green pigment called "phthalocyanine. " Under the right conditions - for example, when heated - these disk-shaped molecules line up like a stack of coins, solidifying as long, rod-like molecular stacks in a well organized film. Chemistry Professor Seth Marder has developed a set of complimentary liquid crystalline materials that also self-assemble into coherent stacks.

The UA team is working to engineer molecules that stack themselves vertically rather than horizontally on the substrate for higher electrical mobility. That is no small feat, Armstrong said, "But we feel a 10 percent conversion efficiency is a realistic goal, based on our own recent work and the work of several other groups in Europe and Japan."

"What you'd really like is a solar panel array that would come on flexible plastic which you could roll out on your roof like wallpaper." - Neal Armstrong

Such a film could greatly improve a potentially important type of organic solar cell, Kippelen said.

Such cells, first proposed in the 1990s by scientists at the Ecole Polytechnique in Lausanne, Switzerland, already have reached power conversion efficiencies of 10 percent. But the solar cells are not widely practical because they contain liquid electrolytes. The electrolytes can evaporate and decompose as they sit in the sun and can be hard to process into large area PV arrays.

The dye sensitized organic solar cell has a transparent electrode coated with a porous network of titanium dioxide nanoparticles - the semiconductor. By itself, titanium dioxide cannot absorb visible sunlight efficiently. So a photosensitive dye is added to the network - the sensitizer. The dye absorbs photons from sunlight and releases electrons that flow as an electrical current to a counter electrode.

But once photoactive dye molecules have given up electrons, they must be very quickly recharged. Dye molecules cannot absorb more photons until their lost electrons are replaced, or "regenerated."

"We want to replace the liquid electrolyte currently used to regenerate dye in these organic solar cells with a new charge-transporting film," Kippelen said. "That would be the big development."

"And we can make the titanium dioxide network even more porous so photoactive dye covers a greater surface area," thereby increasing the solar cell's light absorption potential, he added. " Further, dyes currently used do not absorb the full spectrum of the sun, only the visible part, and only about 45 percent of the light is being harvested. So we also will work on making dyes that absorb more of the infrared part of the spectrum."

"Theoretically, there is no reason that we cannot make organic solar cells with 20 percent efficiency." - Bernard Kippelen

The Department of Energy and Office of Naval Research are funding organic solar cell research through new programs, and other basic and applied research funding sources are increasingly interested in the technology as well, the UA researchers say. They are confident that their team-oriented approach is ideal for developing the technology, they add, and that simple, inexpensive solar cell materials may generate electricty in the not-too-distant future.

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